The present invention relates generally to an impact device, and more particularly to an impact device of the type having a chuck which can releasably accommodate a tool bit.
Impact devices of this general type are already known in the art. They may, for example, be constructed as hammer-drills, as power chisels or the like, and the chuck accomodates a tool in form of a chisel bit, a drill bit or the like. The principle of operation of these devices is that the tool bit is turned in rotation and also has axially acting impacts imparted to it. Some of these devices have tool bits that cannot be removed, but as a general rule these devices are provided with a chuck having an axial guide into which the tool bit can be inserted and from which it can be removed, so that either a different type of tool bit can be used with the device, or else a tool bit that is warn or damaged can be removed and replaced by another one. Usually, the axial guide for the tool bit is in form of a bore in the chuck, having a polygonal cross-section in which a shank of the tool bit is matingly received. Thus, the tool bit cannot turn with reference to the chuck, but it can turn with the chuck and can of course also have axial impacts imparted to it.
It is important in these devices to fix the tool bit so that it cannot come loose, that is that it cannot unintentionally come out of the chuck, or perform unintended axial movements relative to the chuck. For this purpose it is known to provide the tool bit with a recess or an opening, and to insert a pin into this recess, the pin being so configurated that when it is turned about its longitudinal axis through, e.g. 90.degree. or 180.degree., it will permit the tool bit to be removed, or another tool bit to be inserted.
These known devices have certain disadvantages. One of these results from the fact that unavoidably these impact devices will at times perform idling movements, that is if for instance the tool bit has just penetrated a wall on one blow, during the next impact there will be nothing to resist the forward movement of the tool bit, and the impact device will "idle", that is it will exert impacts upon the tool bit which, however, are not resisted by a workpiece against which the tool bit is pressed. Tests that have been conducted with impact devices of the type in question have shown that under these and other circumstances the resting device, for instance the pin that is to hold the tool bit in the shank, are subjected to very high stresses. Of course, they are also subjected to such stresses when the impact device actually performs work, and it follows that these arresting devices are components of the impact devices which are constantly subject to the danger of breakage.